A. Field of the Invention
The invention relates to the field of cushioning, particularly to equalization of pressure across the surface of a cushioned object and minimization of high pressure areas. More specifically, the invention is a plurality of individual cushioning bladders configured and arranged so that each bladder has substantial freedom of movement with respect to any protrusions on the cushioned object, and so that the bladders may interact with and influence each other to achieve even distribution of force and pressure across the cushioned object, regardless of any irregular surface or protrusions existing on the cushioned object. In various embodiments, the cushion may be uniformly shaped, pre-shaped, or periodically reshapable.
B. The Background Art
It is well known that persons who must sit or lie in a reclined position for extended periods of time experience localized tissue breakdown leading to decubitus ulcers (pressure sores), which in turn can lead to extensive hospital stays, and in severe cases, even amputation. The tissue breakdown of decubitus ulcers is caused by lack of blood circulation in localized areas. Blood circulation is slowed or prevented when pressure in the tissue caused by an external source exceeds the internal blood pressure in the capillaries and vessels of the tissue. Such excessive localized external pressure often occurs when a person sits or lies on traditional cushioning devices, which can cause higher pressures on bony prominence (e.g. hip bones) than on non-prominent areas.
Traditional cushioning devices consist of flexible foams which when deformed behave similar to springs. The more the deformation, the more force the foam applies to the deforming object in an effort to return to its original undeformed shape (i.e. the foam has "memory"). When the deforming object on a cushion is a portion of human body, the deforming force causes excessive pressure on body tissue which can lead to discomfort or to decubitus ulcers.
Supporting pressure on the tissues of sitting or lying persons cannot be eliminated, since in order for the person to be supported, the total cushioning force on all of his/her tissues must equal the weight of the person. The objective of an effective cushion is not to eliminate these supporting forces, but to distribute them as evenly as possible to eliminate peak pressures, and to distribute them over as large an area as possible to minimize average supporting pressure. In most cases, and specifically in the cases of wheelchair cushions and bed mattresses, the area of human tissue to which supporting force is applied is large enough that if the cushioning force is equalized over that entire area, the pressure on the human tissue will be less than that which causes decubitus ulcers. The pressure at which circulation is slowed to the point that tissue damage and decubitus ulcers become a substantial danger is 30 millimeters of mercury.
Optimally, a cushion should have a shape that is precisely the complementary shape of the object being cushioned so that it contacts and supports each protrusion and crevice of the cushioned object. This results in the supporting forces being applied to the cushioned object over the largest possible. To area, resulting in the lowest possible supporting pressure. To fully achieve this goal, the cushion material must not be attempting to return to some other shape (i.e. it must not have memory).
A flat foam cushion is very ineffective at achieving these goals because: (1) the cushion is not originally shaped to match the contours of the object to be rested on it, and when an object is placed on the foam cushion, the foam imperfectly re-shapes to the object's contour, not utilizing the entire surface area over which the supporting force is applied; and (2) because of its memory, the foam attempts to rebound and return to its original flat shape, applying stressful pressure to the cushioned object in direct proportion to the degree to which the foam cushion has been deformed. The prominent areas of a human body being cushioned (e.g. the area near hip bones) deform the foam from its original flat shape more so than do other areas of the body, causing the pressure to be very high near the prominent areas in comparison with the non-prominent areas. These pressure peaks can cause discomfort and can cause tissue damage that leads to decubitus ulcers.
Pre-shaping the foam (e.g., cutting it to match a particular body contour) is only marginally effective at achieving equalized pressure distribution because the cutting process is inherently inaccurate, precise placement of the object or person on the contoured foam cushion is difficult, movement of the object or person on the cushion defeats the benefits of the contour, and the memory of even pre-shaped finite-thickness foam causes undue pressure on body tissue and can lead to tissue damage.
In addition to foam cushions, the prior art includes various fluid-filled cushions. Most prior art fluid-filled cushions have been more effective than foam cushions in equalizing supporting pressures. The prior art fluid-filled cushions consist of large single bladders (compartmentalized or single-compartment) filled with a fluid (some type of liquid or gas/air). Some of the prior art bladders are placed atop a shaped tray, the edges of which prevent the fluid from flowing laterally. Fluids are more effective than foam in providing non-damaging cushioning to human tissue because they have relatively Little shape memory, and if properly containerized, they will flow to generally match the contour of the body being cushioned.
All fluid cushions (including the invented cushion described herein) depend on "hammocking" to suspend the person on the cushion fluid within the cushion bladder(s). Hammocking is defined as the tensioning of the top surface of a bladder material by limiting its edge (i.e. side) movements when a force is applied to the top surface of the bladder in the general direction of the fluid beneath. This is similar to the mechanics of a well-known sleeping hammock which has its ends restricted from moving by being tied between two trees, thereby tensioning the hammock to support a person lying on the hammock. If cushions provided no hammocking, the person sitting or lying on the cushion would sink through the fluid in the bladder(s) and bottom out on the surface beneath the fluid-filled bladder(s). This can be proven by the principle of physics that the buoyant (upward) force on an object in a fluid is equal to the weight of the fluid displaced by the object. In order to suspend a person on a fluid cushion without hammocking, the person would have to sink deep enough into the fluid to displace his/her body weight of fluid. This cannot occur in any prior art fluid cushions or the invented cushion, all of which are limited to a few inches in thickness. Thus, the suspension of the person is not entirely from buoyancy in fluid, and is in fact mostly from hammocking of bladder material.
The objective of a fluid-filled cushion, therefore, should not be to eliminate hammocking, but to distribute the hammocking forces over as large an area of the supported object as possible and as evenly as possible. Prior art fluid cushions fail to do this. Single bladder non-segmented cushions of the prior art must stretch the bladder skin tightly (i.e. fill the cushion very full with fluid) to prevent bottoming out. Otherwise the fluid under protruding body parts would flow not just to non-protruding parts (which would in fact help equalize pressure), but also to parts of the bladder on which the person is not sitting, thus allowing the person to sink through the fluid to the surface beneath the cushion. This prior art practice of filling the cushion very full creates a single hammock from edge to edge of the cushion. A single hammock has high peak pressures because it suspends protruding body parts on the bladder material first, placing additional pressure on those protruding parts when the full weight of the person deforms the resistive bladder further, and it does not fully conform to the contours and crevices of the cushioned body. Thus, a single bladder cushion has some characteristics and negative attributes similar to foam.
In an attempt to solve these problems, recent prior art cushioning devices have used a segmented bladder to prevent fluid flow from one segment or cell to another. For example, some prior art bladders are quadrilaterally segmented by sealing the top surface of the bladder to its bottom surface to create four segmented cells in such a way as to prevent fluid from flowing from the forward half to the back half, or from the left half to the right half, or etc. This creates four hammocks, and so distributes load better than a single hammock.
Unfortunately, the prior art methods of segmenting a single large bladder (i.e., selectively sealing the top bladder surface to the bottom bladder surface) created a situation in which the bladder surfaces, and hence also the fluid, were movement-restricted by these segmenting seals and could not fully conform to the irregular surface of the user's body. This reduced the total surface area of human tissue onto which the cushioning force was applied, thus raising the average pressure and increasing the danger of tissue damage. Also, the small number of bladder segments used by the prior art created some peak pressure areas. Additionally, because this cushion design prevented fluid from flowing from one segment or cell of the cushion to another, pressure equalization among cells was not achieved and the danger of decubitus ulcers from high pressure spots was not eliminated.